There are various types of mechanical systems in which it is necessary to accommodate torsional motion while providing resistance to axial, flexural and transverse loading. The torsional motions and loads may be applied either statically or dynamically in various operating environments.
In some mechanical applications, connecting links fabricated from tubes have been used to transmit such torsional motion while sustaining such loads. While hollow tubes are axially and transversely strong, and flexurally stiff, they are also generally quite stiff in torsion, depending upon the material of which they are made, their wall thicknesses, diameters, lengths, etc. With such tubes, long lengths are often required if substantial torsional deflection, opposed to torque transmission, is desirable between the opposite ends of the tube.
Devices for use in transferring and dampening torsional loads are known. An example of such a device may be found in U.K. Pat. No. 748,587. In the disclosed device, a series of curved metal tube sections are clamped together in a stack in frictional engagement with one another to dampen torsional loads applied at opposite ends of the stack of tube sections. While this device may function satisfactorily in certain applications, it is not suited for those applications requiring the accommodation of substantial angular deflections for prolonged periods of time.
A particularly severe application for a connecting link exists in conjunction with mounting the rotors of a helicopter to its hub. Customarily, the rotors are connected to the hub by various means designed to resist various combinations of ultimate and cyclic loads, such as described above, resulting from the rotation of the rotor and its in-flight pitch changes. Commercially satisfactory links must be capable of withstanding various loads for prolonged periods of time without failure under a wide range of environmental conditions involving operation over a wide range of temperatures and under various corrosive conditions.
Over the years, several types of rotor retention structures have been proposed. Early retention structures included a series of laminated straps connected between the hub and the rotors. An example of such construction may be found in U.S. Pat. No. 2,961,051 (Wilford). Some helicopters have been manufactured using so-called wire wound tension torsion tie bars which comprise a plurality of fine wires wrapped around bushings and bonded together in an elastomeric matrix. An example of such a link may be found in U.S. Pat. No. 3,520,637 (Tobey). Yet other helicopters have utilized so-called elastomeric bearings to connect rotors to hubs. An example of such a construction may be found in U.S. Pat. No. 4,142,833 (Rybicki).
While the various types of rotor retention systems such as disclosed in the aforementioned patents may function satisfactorily for the particular helicopter application for which they have been designed, there is a need for a compact yet lightweight link which is capable of withstanding the various loads which are applied between the rotor of a helicopter and its hub while permitting rotor pitch changes to be made. For instance, such a link must have substantial axial strength in order to withstand the applied centrifugal forces; it must have substantial flexural modulus in order to resist bending moments caused by lift, as well as lead-lag loads on the rotor; and it must be capable of accommodating cyclic torsional motion between the hub and the rotor for flight control and trim purposes. In addition, a satisfactory link must be resistant to fatigue and must be capable of operating effectively in hostile environments.